Methods and devices of the aforementioned type are known in many forms, for example, from DE 199 29 199 A1. Three-dimensional objects are therein produced by means of layer by layer curing or melting of preferably plastics materials by means of digital mask exposure wherein the energy input per unit area is controlled by means of the exposure time, according to the locally available light power.
A variety of problems to be solved arise in the production of three-dimensional objects by means of these methods and devices. In particular, it is important, for example, to homogenise a light intensity distribution. As a result of an optical system that is used which comprises, in particular, a radiation source and a projection lens system, typically an inhomogeneous intensity distribution comes about during the imaging. Such an inhomogeneity should however be prevented as far as possible so that the intensity distribution is as homogeneous as possible in a construction-projection plane in which the three-dimensional object is formed.
It is also important to achieve an even hardening depth with differently configured cross-sectional areas of the object layers of the object. Thus, firstly, structures in the object layer with a delicate structure or cross-sectional area are hardened less deeply than structures with a large cross-sectional area. Furthermore, a so-called overexposure can occur in the contour region of the object layer. This can have the consequence, in particular, that for example small hollow spaces in relatively massive structures are not formed at all due to the overexposure of contours thereof since, due to the overexposure, the material in the contour region hardens as far as into the hollow space that is to be formed. And finally, the so-called raw-body hardness is often not sufficient during the production of the three-dimensional object. This is to be understood, in particular, to mean that with particularly delicate (supporting) structures, only inadequate hardening can occur during production of the object, so that these delicate structures often tear off. It can also occur that with very delicate structures, the critical energy for hardening is not achieved and thus the delicate structures are not generated. In the case of massive components, it may be desired to overexpose the inner region during the construction process in order to achieve a greater raw body hardness in the component, since during the burning process, particularly with opaque materials, the radiation has only a small penetration depth.
In order, particularly, to achieve a more even hardening depth with differently configured cross-sectional areas, it is known from the prior art by means of grey value control of the individual pixels of the mask or by means of a multiple exposure using a plurality of masks for a single object layer, to achieve a controlled, pixel-accurate hardening of the object or the object layers forming said object. Reference is made to the aforementioned DE 199 29 199 A1 with regard to grey value control and to EP 1 849 586 A1 with regard to multiple exposure. A disadvantage of grey value control is, in particular, that due to the destruction/reduction of radiation intensity, a longer exposure time is always required per exposure cycle of an object layer in order to achieve the required energy input per object layer. The reason for this is, in particular, that for optimisation, an energy input is always oriented toward the lowest intensity values in the image. In the case of multiple exposure, a plurality of masks must be provided and exposed one after another. This procedure also has the consequence of prolonging the exposure time.